1. Field of the Invention
This invention relates to devices having repetitive layers and, in particular, to devices containing doped repetitive layers.
2. Art Background
A variety of devices are fabricated with repetitive material layers. For example, surface emitting laser diodes (SELDs) are formed from an active diode region and from adjacent multi-layered distributed Bragg reflectors. Each Bragg reflector is typically composed of a series of between 20 to 30 alternating compositional layers. The assemblage of layers constituting one reflector on one side of the active diode is doped n-type to contact the diode n-type region and the opposing second reflector is doped p-type to contact the p-type region. The materials comprising the alternating layers are chosen to have substantially different refractive indices to yield high reflectivity, and are doped to yield desired optical and electrical properties. For example, in the case of a SELD the voltage to drive the laser is conveyed through contacts to the outermost reflector region and emitted light traverses the reflector. Therefore, the doping concentration must be sufficiently high to yield a relatively high electrical conductivity, but not so high as to cause the assembly to absorb sufficient light significantly to reduce reflectivity.
Typically, for SELD devices to yield adequate optical and electrical properties, the first 1 to 5 layers (inner layers) adjacent the active region have relatively low doping (10.sup.17 cm.sup.-3 to 10.sup.18 cm.sup.-3 majority carrier concentration) and the remaining area (outer layers) is intended to be fabricated with relatively high dopant concentrations, i.e., above 10.sup.18 cm.sup.-3. The lower doping concentrations in the center of the diode (inner layers) insure that, in the region where approximately 80% of the light is confined, the free carriers do not cause absorption of more than 1% of the emitted light. Similarly, the intended dopant concentration in the remaining layers is employed in an attempt to maintain a total series resistance in the distributed Bragg reflector of less than 10.OMEGA..
Despite these intentions, in actuality the p-type mirror in a SELD device typically has a series resistance of 100.OMEGA. or higher. This series resistance significantly constrains the voltage which must be applied to operate the device. That is, for voltages greater than 2 volts (for gallium arsenide/aluminum gallium arsenide lasers), sufficient heat is evolved to unacceptably limit device lifetime. This upper limit on voltage in turn undesirably limits the amount of useful light emitted by the laser. Attempts have been made to reduce the resistance of the reflectors by using a graded rather than an abrupt transition between adjacent semiconductor layers of the reflectors [see for example, Tai et al, Appl. Phys. Lett. 56, 2496 (1990)]. Despite these efforts, SELD still have unacceptable parasitic resistance, i.e., resistance greater than 100 ohms and total diode voltage, i.e., voltage greater than 2.0 V.
Thus, the commercial application of SELD devices has been substantially reduced. In particular, SELD devices are considered of particular interest for communication purposes because of their small size and relative fabrication ease (no cleaving necessary). However, the limited output intensity obtainable from such devices as a result of high series resistance, requires a relatively short distance between optical signal regenerators (repeaters) with an associated significant increase in cost. Optical transmission systems using SELDs are therefore presently not commercially desirable. Thus, such devices despite their promise are not presently used in communication systems.